Various methods have been used in the past to attempt to provide electrostatic discharge and overvoltage protection for high bandwidth amplifiers. One of these methods has been to use a diode bridge as a signal limiter in the main signal path, as shown in FIGS. 1 and 2.
FIG. 1 shows one prior art input protection circuit utilizing a full diode bridge as the signal limiter. In this circuit the signal limiter is positioned after the input termination resistor and external to the integrated circuit containing the amplifier to be protected. FIG. 2 shows another prior art input protection circuit utilizing a full diode bridge as the signal input limiter. However, in this circuit the limiter is placed between the input transmission line and the input termination resistor, and both the limiter and the termination resistor are external to the integrated circuit containing the amplifier to be protected.
Putting the termination resistor in front of the signal limiting diode bridge, as shown in FIG. 1, has produces at least four advantages: 1) the input impedance remains nearly the same when an overload signal causes the diode bridge to disconnect the input signal from the positive input terminal of the amplifier, 2) the standing current in the diodes can be relatively low because the amplifier's input impedance is high, 3) the attenuation caused be the bridge is low (again because the amplifier's input impedance is high), and 4) the offset voltage applied to the amplifier's negative input has little effect on the linearity of the signal input limiting diode bridge. This arrangement also produces two disadvantages: A) some of the bandwidth is lost because of the series resistance of the diodes and because of the capacitances associated with the diode bridge and its biasing components, and B) noise is increased because of the increase in resistance in series with the amplifier's positive input.
Putting the termination resistor after the signal limiting diode bridge, as shown in FIG. 2, improves the attainable bandwidth, at least if the resulting combined impedances can successfully be kept close to the characteristic impedance of the input transmission line. Keeping the combined impedances matched with the characteristic impedance of the transmission line bandwidths above 1 GHz typically requires implementing the diode bridge in hybrid circuitry, and doing so increases the costs involved. Moreover, this approach has several other drawbacks: 1) the signal attenuation is increased because the resistance of the diode bridge in series with the termination resistor forms a voltage divider that reduces the voltage across the termination resistor alone, 2) larger bias currents are required to minimize the voltage attenuation and non-linearity caused by the impedance of the diode bridge, and these larger bias currents increase power consumption and cause related problems, 3) overvoltages that cause the signal limiting diode bridge to disconnect the signal path also produce relatively large reflections because the termination resistor is also disconnected along with the amplifier, and 4) there is a reduced range of offset voltages that can be effectively applied to the negative input of the amplifier without causing non-linearity in the effects caused by the diode bridge.
What is desired is a way to protect high bandwidth amplifiers from electrostatic discharge and overvoltage without significant degradation of bandwidth, and without an increase in noise or the voltage standing wave ratio (VSWR), and without adversely affecting the power consumption of the overall circuit.